1. Field of the Invention
The present invention relates to a process for the production of 1,5-naphthylenediisocyanate. More specifically, it relates to a process for producing 1,5-naphthylenediisocyanate at high yields, with little by-products and without the necessity of handling substances poisonous to a human body except chlorine.
1,5-Naphthylenediisocyanate is useful as a raw material for producing a polyurethane elastomer excellent in heat resistance, weather resistance and durability against repeated bending.
2. Description of the Prior Art
The 1,5-naphthylenediisocyanate production method employed in present industry uses naphthalene as a raw material and employs reaction steps including mononitration, dinitration, reduction and phosgene formation.
However, the above method involves various problems. That is, although naphthalene as a starting material is available at a low cost, there are defects that the yield in each step is low and that a large amount of by-products (waste products) are formed. For example, in the dinitration step, 1,8-dinitronaphthalene which is an isomer of 1,5-dinitronaphthalene is formed in an amount 1.8 times the amount of 1,5-dinitronaphthalene. In view of environments and safety, further, the problem is that it is required to handle 1,5-dinitronaphthalene and 1,5-diaminonaphthalene which are intermediates having mutagen poisonous to a human body, and another problem is that it is required to use highly toxic phosgene as a side raw material. In view of product quality, a large amount of hydrolysable chlorine is contained in 1,5-naphthylenediisocyanate and has detrimental effects on the weathering properties and heat resistance of polyurethane products.
Further, a variety of methods have been proposed with regard to the production of aromatic carboxylic acid amides from aromatic nitrites.
For example, Org. Syn. Col. Vol. 2, 586-587 (1943) discloses a method of amidation of o-tolunitrile by using hydrogen peroxide. However, this method uses a large amount of hydrogen peroxide as a side raw material.
British Patents 1133013 and 1351530 describe a method of amidation of a nitrile compound in the presence of manganese dioxide as a catalyst. In this method, a large amount of the catalyst is required, and formed aromatic carboxylic acid amides precipitate as a crystal, so that there is caused a problem that the crystal blocks the active center of the catalyst.
U.S. Pat. No. 3,763,235 discloses a method of amidation of a nitrile compound with a lower aliphatic carboxylic acid in the presence of a metal salt. The method requires a large amount of the catalyst as well. Further, a precipitating crystal of aromatic carboxylic acid amides entraps the catalyst, and the problem is that the separation of these is difficult.
PCT International Publication WO90/09988 discloses a method of contacting aromatic nitrites to perboric acid alkali metal salt in a water-containing alcohol. In this process, expensive perboric acid alkali metal salt is required in a large molar amount, as large as 2.5 to 4 times the molar amount of the aromatic nitrites.
JP-A-6-116221 and JP-A-6-128204 disclose a method of amidation of aromatic nitrites in a water-containing alcohol in the presence of an inorganic strong base and a method of reacting aromatic nitrites with an alcohol in an inorganic strong base to synthesize an iminoether compound and then amidating it by adding water. These Publications include examples of 2,6-naphthylenedinitrile. These methods are industrially excellent in the use of a less expensive alkali metal hydroxide as a catalyst. When the present inventors applied these methods to 1,5-naphthylenedinitrile having lower reactivity than 2,6-naphthylenedinitrile, however, 1,5-naphthylenedicarboxylic acid amide was obtained only at low yields, and it was found that the reaction product contained a non-negligible amount of terephthalic acid formed as a by-product (see Comparative Example 1).
The above carboxylic acid is consumed or converted to a salt of the carboxylic acid with the inorganic strong base which is added to the reaction system. When an isocyanate compound is synthesized from a carboxylic acid amide containing a carboxylic acid or a salt of a carboxylic acid according to the present invention, an end product comes to contain the carboxylic acid or the salt of a carboxylic acid, and it is therefore required to separate the carboxylic acid amide and the carboxylic acid from each other.
As described above, the above conventional methods for producing aromatic carboxylic acid amides from aromatic dinitrils are not necessarily satisfactory from the viewpoint of industry, since there are involved problems in the use of the expensive side raw material and a large amount of the catalyst and a problem that a purification step is required since a large amount of the carboxylic acid is formed as a by-product.
Further, the following methods are disclosed as a method in which aromatic carboxylic acid amides are reacted with chlorine to form aromatic carboxylic acid-bis-N-chloroamides and the aromatic carboxylic acid-bis-N-chloroamides are reacted in Hofmann rearrangement in an alcohol to obtain aromatic carbamate esters.
JP-A-5-65259 discloses a method in which aromatic carboxylic acid amides are reacted with chlorine in methanol to obtain aromatic carboxylic acid-bis-N-chloroamide. JP-A-7-291910 discloses a method in which aromatic carboxylic acid amides are chlorinated by mixing them with a solvent in which chlorine is pre-dissolved, to obtain aromatic carboxylic acid-bis-N-chloroamide. JP-A-5-65266 discloses a method in which aromatic carboxylic acid-bis-N-chloroamides are reacted with an alcohol in the presence of an inorganic strong base to obtain aromatic carbamate esters.
JP-A-5-65266 describes Example in which sodium hydroxide was added to, and mixed with, methanol under atmospheric pressure, 2,6-naphthalenedicarboxylic acid-bis-N-chloroamide at 5.degree. C. was added little by little over 30 minutes while retaining a temperature of 5 to 10.degree. C., and then, the reaction solution was gradually temperature-increased and allowed to react under the reflux of methanol for 2 hours, followed by cooling, filtering, washing with water and drying, to give 2,6-bis(methoxycarbonylamino)naphthalene having a purity of 95.4% by weight at a yield of 94.8 mol %.
The present inventors applied the above method to 1,5-naphthalenedicarboxylic acid-bis-N-chloroamide and found the following. The yield of 1,5-bis(methoxycarbonylamino)naphthalene was 49.2%. That is, in the case of 1,5-bis(methoxycarbonylamino)naphthalene, no high yield can be achieved unlike the high yield of 2,6-bis(methoxycarbonylamino)naphthalene (see Comparative Example 2).
There is also a method of pyrolysing aromatic carbamate esters to form isocyanates. This method includes a gas phase method and a liquid phase method, and the liquid phase method is advantageous in industry. The pyrolysis of carbamate esters undergoes in a reversible reaction, and its equilibrium shifts toward the formation of an isocyanate at a high temperature. The pyrolysis is generally therefore carried out at high temperatures. Since, however, the condition of the reaction is severe, an isocyanate causes various irreversible side reactions and easily form, for example, ureas, amides, carbodiimides, urethidiones and isocyanurates. These side reactions not only decrease the selectivity to isocyanates, but also cause the formation of by-products having high boiling points, so that it is made difficult to carry out continuous operation for a long period of time by the clogging of a reactor and tubings. For overcoming these problems, there have been proposed a method of using a catalyst or a solvent, a method of withdrawing a formed isocyanate from a reaction system in a short period of time by reaction distillation, and the like.
As a general method of pyrolyzing carbamate ester, for example, JP-B-57-45736 discloses method using a catalyst prepared by dissolving one or more metals selected from the group consisting of metal atoms belonging to the groups IB, IIB, IIIA, IVA, IVB, VB and VIII of the periodic table or compound(s) thereof in a solvent. JP-A-54-88201 discloses a method using an alkaline earth metal and an inorganic compound thereof as catalysts. JP-A-57-158747 discloses a method using one or more simple substances or compounds selected from the class consisting of element simple substances belonging to the copper group, the aluminum group, the carbon group excluding carbon and the titanium group of the periodic table and oxides or sulfides of these, as a heterogenous catalyst in a solvent. JP-A-7-258194 discloses a method using organic sulfonic acid and an alkali metal salt thereof as catalysts in a solvent.
For obtaining isocyanates at high yields by preventing the formation of by-products in the pyrolysis of carbamate esters, there are also proposed methods using stabilizers. For example, JP-A-57-123159 discloses a method using carboxylic acid chloride, sulfonate ester and an alkylating agent. JP-A-1-125359 discloses a method using phosphorous triester. JP-A-9-87239 discloses a method using aromatic sulfonic acids or aromatic sulfone amides.
Various methods are also proposed for the pyrolysis of bis(alkoxycarbonylamino)naphthalene. JP-A-56-65857 describes Example in which a tubular reactor packed with zinc chips was provided, and 1,5-bis(ethoxycarbonylamino)naphthalene was pyrolyzed under reduced pressure at 350.degree. C., to give 1,5-naphthylenediisocyanate at a yield of 80.5%. JP-A-2-295958 describes Example in which 1,5-bis(ethoxycarbonylamino)naphthalene was pyrolyzed in the co-presence of a sulfolane solvent and a dibutyltin dilaurate catalyst under atmospheric pressure at a temperature of 200.degree. C. for 5 hours, to give 1,5-naphthylenediisocyanate at a yield of 80.6%.
JP-A-11-5773 describes Example in which 2,6-bis(methoxycarbonylamino)naphthalene was pyrolyzed in two reaction solvents and one collecting solvent in the co-presence of a dibutyltin dilaurate catalyst and p-toluenesulfonic acid as a stabilizer under reduced pressure at 250.degree. C., to obtain 2,6-naphthylenediisocyanate at a yield of 93.5%. When the present inventors applied this method to the production of 1,5-naphthylenediisocyanate, the yield of the 1,5-naphthylenediisocyanate was 63.9%, and it has been found that 1,5-bis(methoxycarbonylamino)naphthalene is less reactive than 2,6-bis(methoxycarbonylamino)naphthalene, so that no high yield can be attained.
As described above, when 1,5-bis(alkoxycarbonylamino)naphthalene is pyrolyzed according to any conventional method, the yield of 1,5-naphthylenediisocyanate is 81% or less, which is not at all satisfactory in industry.
As explained above, the existing methods of producing 1,5-naphthylenediisocyanate involves various problems, and it is desired to develop a new process.